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Why do We need Buffers? Because it saves us a lot of trouble! The minimum size of erase/programming is one page. You will have to send exactly one page data through SPI for every write operation without buffers. You may have to first send back one page data from flash even before that!

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Timing Issues Erase and Program time Flash output delay Setup time, hold time of all serial port … You are able to find timing diagrams and characteristic tables in datasheet. (Figure Credit: AT45DB161D Datasheet)